Photochemical smog does not contain .....

$DDT$ is a

Which of the following is not a pollutant?

Thermal pollution of water bodies is due to

In a eutrophic water body,the excessive growth of $\underline {X}$ leads to the death of $\underline {Y}$.

Is it true that if the dissolved oxygen level drops to zero,the water will become septic? Give an example which could lower the dissolved oxygen content of an aquatic body.